1. Field of the Invention
This invention relates to a wet friction member such as a wet clutch and a lock up friction member that generates a torque by applying a high pressure to an opposite surface of a separator plate from the wet friction member, while being immersed in oil, and a method for manufacturing the same wet friction member.
2. Description of the Related Art
A wet friction member has a basic structure composed of a combination of a plurality of wet friction plates having wet friction members fixed on respective steel plates and a plurality of separator plates as counterpart members. Each of the wet friction members is made by blending organic fibers with synthetic resin and friction conditioner or modifier (organic component and/or inorganic component) and the like and thermally hardening or curing it. The separator plate is engaged with a friction surface of the wet friction member. The wet friction plates and the separator plates are engaged with each other in a lubricating oil or an automatic transmission oil (ATF) to transmit power, while they are disengaged with each other to release or cut off the power.
As an example, Japanese Patent Laid-Open Publication No. 2005-42864 discloses a wet friction member. In the invention described in the Japanese Laid-Open Publication, the wet friction member comprises a bottom part to be a base of the wet friction member and an upper part on a side in contact with a separator plate of a counter member, wherein the bottom part and the upper part have a high hardness and a low hardness, respectively. The wet friction member has a friction surface in contact with the counter member. It is written in the publication that the friction surface of the wet friction member is soft and fits well with the counter member. Moreover, it is written in the publication that an oil film formed between the wet friction member and the separator plate of the counter member can be reduced and that the wet friction member sufficiently exhibits proper characteristics or properties originated from various components blended in the wet friction member.
Moreover, according to the publication, since the wet friction member as a whole has the bottom part as the base being hard, it is prevented from being soft and from causing runout or slippage. The whole friction member firmly presses the separator plate of the counter member to reduce the oil film between the friction member and the separator plate. Thus, the wet friction member exhibits original characteristics and properties and has a high friction coefficient μ and an improved positive μ-V gradient.
As described above, a wet clutch, a brake friction member, a lock up friction member or the like is connected or disconnected with a counter plate by applying a high pressure from the friction surface of the wet friction member to the facing or opposite surface of the counter plate, while being immersed in the ATF. When such a wet friction member is engaged to generate a torque, the ATF is present on the friction interface of the wet friction member. At this time, the ATF has a role of absorbing heat generated by friction. On the other hand, it is a current situation that the presence of the ATF causes a lower friction coefficient μ.
However, as needs of automobile manufacturers and consumers, there is high demand for reduction in fuel consumption based on weight reduction. It is also an inevitable requirement for the wet friction members to reduce the number of the members. Then, it is required for the wet friction member to reduce its weight and to have high efficiency. In order to satisfy such a requirement, the friction coefficient μ should be increased during engagement. In principle, a material having a higher friction coefficient μ should be added to increase the friction coefficient μ of the wet friction member.
An existing common wet friction member is composed of base fibers such as pulp and/or aramid fibers and a filler such as a friction conditioner and an extender pigment. The base fibers and the filler are formed into a paper-made substrate by a paper making method. The paper-made substrate is impregnated with a phenolic resin which is a thermosetting resin and heated and cured. The wet friction member has a thickness of 1 mm and a hardness of approximately 50 in Rockwell hardness measured based on JIS (Japanese Industrial Standard) S-7726. In addition, an amount of deformation is approximately 150 μm.
A friction test was performed on the wet friction member. In detail, there were various differences in the materials. However, very few of them exhibited a friction coefficient μ the exceeded a value close to, but not exactly equal to, 0.150 to 0.160.